A well-known use for thermoelectric devices is for the extraction of electric power from waste heat. For example, U.S. Pat. No. 6,527,548 discloses a self powered space heater for a truck in which heat energy for the heater is used to power electric components of the heater plus charge a battery. In U.S. Pat. No. 6,053,163 heat from a stovepipe is used to generate electricity. U.S. Pat. No. 6,019,098 discloses a self-powered furnace. Various types of thermoelectric modules are available. A very reliable thermoelectric module with a gap-less egg-crate design capable of high-temperature operation is described in U.S. Pat. Nos. 5,875,098 and 5,856,210. U.S. Pat. No. 6,207,887 discloses a miniature milliwatt thermoelectric module useful in space applications in combination with radioactive heat source. Quantum well very thin layer thermoelectric modules are known. Some are described in U.S. Pat. Nos. 6,096,965, 6,096,964, 5,436,467 and 5,550,387. U.S. Pat. No. 6,624,349 describes an electric generator using a thermoelectric module to generate electric power from the heat of fusion produced by the freezing of a phase change material. All of these patents referred to above are assigned to Applicant's employer and they are all incorporated herein by reference.
Workers in the thermoelectric industry have been attempting too improve performance of thermoelectric devices for the past 20–30 years with not much success. Most of the effort has been directed to reducing the lattice thermal conductivity (K) without adversely affecting the electrical conductivity or Seebeck coefficient. Experiments with superlattice quantum well materials have been underway for several years. These materials were discussed in an paper by Gottfried H. Dohler which was published in the November 1983 issue of Scientific American. This article presents an excellent discussion of the theory of enhanced electric conduction in superlattices. These superlattices contain alternating conducting and barrier layers and are believed to create quantum wells and they do in fact improve electrical conductivity. These superlattice quantum well materials are crystals grown by depositing semiconductors in layers each layer with a thickness in the range of a few to up to about 100 angstroms. The dimensions of atoms are in the range of an angstrom; therefore, each layer is only a few atoms thick. (These quantum well materials are also discussed in articles by Hicks, et al and Harman published in Proceedings of 1992 1st National Thermoelectric Cooler Conference Center for Night Vision & Electro Optics, U.S. Army, Fort Belvoir, Va. The articles project theoretically very high ZT values as the layers are made progressively thinner.) The idea being that these materials might provide very great increases in electric conductivity without adversely affecting Seebeck coefficient or the thermal conductivity. Harmon of Lincoln Labs, operated by Massachusetts Institute of Technology has claimed to have produced a superlattice of layers of (Bi,Sb) and Pb(Te,Se). He claims that his preliminary measurements suggest ZTs of 3 to 4.
Most of the efforts to date with superlattices have involved alloys that are known to be good thermoelectric materials for cooling, many of which are difficult to manufacture as superlattices. A large number of very thin layers (in the '467 patent, about 250,000 layers) together produce a thermoelectric leg 10 about 0.254 cm thick. In the embodiment shown in the figures all the legs are connected electrically in series and otherwise are insulated from each other in an egg-crate type thermoelectric element. Negative charges flow from hot to cold through n-legs and positive charges flow from cold to hot through p-legs; therefore, current flows from the hot side to the cold side through P legs and from the cold side to the hot side through N legs. These patents disclose superlattice layers comprised of: (1) SiGe as conducting layer and Si as a barrier layer and (2) alternating layers of two different alloys of boron carbide. In the '387 patent Applicants disclose that they had discovered that strain in the layers can have very beneficial effects on thermoelectric properties of the elements disclosed in the '467 patent.
Vehicle tracking devices are known. These are typically radio transmitters that are attached to a vehicle. In many cases the tracking devices transmit a location signal to an earth satellite that in turn relays the signal to a station on earth. An owner of a fleet of trucks my install such a transmitter on its trucks so that the location of each of its trucks is known at the Owner's offices at all times. In some cases the transmitters are installed covertly so that the driver of the vehicle is not aware of the transmitter. Police or other crime fighters may use such tracking devices as a method for monitoring the movements of suspected criminals. These tracking devices are normally battery powered and transmission from the devices cease when the battery is discharged.
What is needed is a better technique for providing electric power for vehicle tracking devices.